Maybe it's a bad e-mail on yor ISP's mail server? Does your ISP allow access to your e-mail using an http webpage? If so, try reading the mail from the mail server and then delete the mail.
See
http://bugzilla.mozilla.org/show_bug.cgi?id=166111
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------- Additional Comment #1 From Jason Airlie 2002-10-04 06:08 -------
Getting the same thing on Win XP Mozilla build 2002093010. Plenty of disk space and I am logged in with Administrator rights. I have gone so far as to delete all the files for that account. Mozilla recreates the mailboxes and still gives the error. No filtering prior to Mozilla happening here.
Could it be a 'bad' email message that is causing the error?
------- Additional Comment #2 From Jason M. Waldo 2002-10-07 21:13 -------
I believe it to have been a bad email on the server. I was able to access the messages through an HTML interface and delete them one at a time until they would all download.
------- Additional Comment #3 From Jason Airlie 2002-10-08 05:14 -------
Same here, I deleted some spam off the server and was then able to download the rest of my mail.
------- Additional Comment #4 From Colin Blake 2002-10-10 10:44 -------
(snip)
This problem just happened to me, though I'm running Netscape7 on Windows98.
Each time I tried to "Get New Mail" just one message would come down (always the same one) and I'd get the Alert message "Unable to write the email to the mailbox. Make sure the file system allows you write priveleges, and you have enough disk space to copy the mailbox." Once I manually deleted the message from the POP3 server, everything was fine again.
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UserAgent: Mozilla/5.0 (Windows; U; Win95; en-US; rv:1.0.1) Gecko/20020826